Abstract

Stratiform, fine-grained pyrite is regarded as an indicator of early diagenetic, iron sulfide mineralization in sediments. It is further regarded as having formed at an early stage, by which it is associated with many stratiform lead-zinc ore deposits throughout the world. Examination of the fine-grained stratiform pyrite at the Cu-Pb-Zn mine at Mount Isa, Queensland, however, indicates that at least at this locality, it formed at a much later stage.A textural characteristic of the Mount Isa pyrite is that it normally overprints two generations of carbonaceous cleavage planes. The first of these (S 2 ) is subparallel to bedding and the second (S 3 ) is axial plane to the mesoscopic folds in the mine. The density of pyrite deposition is controlled by zones defined by these cleavages, commonly in association with neomorphic dolomite in adjacent laminae. In correlated finely laminated sequences, the pyrite appears to have overprinted the near bedding-parallel cleavage after complete compaction of the laminite. Most of the fine-grained pyrite is closely associated with other sulfides (sphalerite, galena, and chalcopyrite), although it generally forms in layers adjacent to them. Where there is an overprinting relationship shown between the fine-grained pyrite and other sulfides, the pyrite is paragenetically earlier and either overgrown, or replaced, by the economic sulfides.The overall distribution of fine-grained pyrite not only suggests a strong spatial association with the Cu-Pb-Zn system, but it continues outward, both in the southerly and northerly direction, to form a halo to it. The gross distribution of fine-grained pyrite strongly crosscuts the stratigraphy, with its maximum development around the center of the mine and a progression of concentration to higher stratigraphic levels in the north. This is the same crosscutting direction as the silica-dolomite boundary, the terminations of the lead-zinc orebodies, and the copper ore. In addition, a pyrite envelope occurs around the 3000/3500 copper orebodies in the northern footwall.Distribution of sulfur isotope ratios in fine-grained pyrite, on both the microscopic scale using the ion microprobe and also on the scale of deposits using conventional methods, has been used as a major argument for early diagenetic deposition. In particular, the large spread of delta 34 S values at the HYC deposit, McArthur River, is presumed to result from biogenic sulfate reduction, necessitating depositional processes operating during early diagenesis. This spread is not greatly different from that of the associated economic sulfides, which were interpreted as hydrothermal, suggesting that the fine-grained pyrite was itself hydrothermal. This alternative interpretation may also apply at Mount Isa, where a similar relationship is observed between the sulfur isotope systematics of pyrite and other sulfides.Mount Isa fine-grained pyrite is therefore interpreted to have formed late in the main deformational event that produced mesoscopic structures in the mine sequence, probably by abiogenic sulfate reduction. Thus pyrite formation was ultimately controlled by the same structures as those that controlled the Pb-Zn ores during formation of the S 3 cleavage, which are the Paroo-Basement fault system and shear along bedding in well-laminated carbonaceous siltstones.

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